There are eight groups of normal elements (IA, IIA, IIIA, IVA, VA, VIA VIIA VIIIA) in the periodic table. Group VIIIA consists of the noble gases or zero group elements because they are all very stable and chemically inert under ordinary conditions. They exist in atomic form in the atmosphere. They have a general electronic configuration = ns2 np6 (8 electrons in the valence shell) except He (1s2). These noble gases have completely filled valence shells (s and p subshells). Their octet is complete, so they do not participate in ordinary chemical reactions and are called inert gases. They have eight electrons in their valence shell, except He, which has two electrons in its valence shell.
He = 1s2 Ne 1s2, 2s2, 2p6 Ar 1s2, 2s2, 2p6, 3s2, 3p6
In 1916, a chemist G. N. Lewis used the concept of octet (eight electrons) and duplet (2 electrons) electronic rule to explain the reactivity and stability of molecules.
The octet rule states that an atom is most stable when its valence shell contains eight electrons. This principle is derived from the observation that atoms of the major group elements tend to participate in chemical bonding in the form of eight electrons per atom in the resulting molecule. This rule only applies to the major group element. The chemical behaviour of the main group elements can be predicted with the help of the octet rule. This is because the rule only involves s and p electrons. Molecules such as oxygen, nitrogen, and halogens follow the octet principle. Hydrogen, helium, and lithium follow the duplet rule because their electrons lie in the s orbital.
Na 1s2, 2s2, 2p6, 3s1 (unstable, reactive, incomplete octet) Loss of one electron Na 1s2, 2s2, 2p6 which is the same as that of Ne Cl = 1s2, 2s2, 2p6, 3s2, 3p5 (unstable, reactive, incomplete octet), Cl = 1s2, 2s2, 2p6, 3s2, 3p6 which is the same as that of Ar.
The tendency of atoms to acquire two electronic configurations in their outermost shell during bond formation is called the duplet rule. They attain electronic configuration like helium.
For Example
Li = 1s2, 2s1 lose 1 electron to form Li+ (1s2)
Be = 1s2, 2s2 loses two electrons to form Be2+ (1s2) Helium has two electrons in its valence shell and is also chemically inert. Some elements that are close to He on the periodic table tend to achieve two electronic configurations in their valence shell. For example, hydrogen, lithium, and beryllium, etc., tend to achieve two-electron configuration in the valence shell.